A heat exchanger is a device for moving heat from one fluid to another (i.e., from a warm or hot fluid to a cold or cooler one) without allowing the fluids to mix. A heat exchanger typically consists of a series of tubes in which one of the fluids runs. The second fluid runs over the tubes and is heated or cooled. Evaporators, condensers, radiators, and the like are heat exchangers. For example, refrigeration systems, i.e., air conditioners, typically include two heat exchangers, usually referred to as the evaporator and the condenser.
FIG. 1 shows a simplified block diagram of an exemplary refrigeration system 20. Refrigeration system 20 includes a compressor 22 in fluid communication with a condenser 24 via a fluid line 26. Condenser 24 is in fluid communication with a metering device 28, which may be in the form of a restrictor or an expansion valve, via a fluid line 30. Metering device 28 is coupled with an evaporator 32 via a fluid line 34, and a fluid line 36 connects evaporator 32 to an input of compressor 22.
In operation, relatively high pressure refrigerant, denoted by arrows 38, is discharged in a gaseous form from compressor 22 via fluid line 26 to condenser 24. At condenser 24, refrigerant 38 is condensed by the action of a cooler fluid, such as air, flowing through condenser 24. The liquid refrigerant 38 thus formed flows via fluid line 30 to metering device 28. Metering device 28 controls the pressure and flow of refrigerant 38 into evaporator 32 in accordance with need. Refrigerant 38 passes into fluid line 34 and flows at relatively high velocity through fluid line 34 and into evaporator 32. Air, as denoted by an arrow 40, may be either blown or drawn through evaporator 32. As air 40 passes through evaporator 32, evaporator 32 removes heat (i.e., cools) air 40. The cooled air 40 is subsequently returned to the area to be cooled, for example, air 40 may be returned to a vehicle compartment. Warmed refrigerant 38 exits evaporator 32 and is returned via fluid line 36 to compressor 22 where the refrigeration cycle is continued.
To obtain the maximum heat transfer from air 40 to refrigerant 38, refrigerant 38 may be routed in evaporator 32 to make multiple passes through the air stream to be cooled, prior to being discharged from evaporator 32 for recirculation. Indeed, evaporators may be categorized in accordance with the number of times cold refrigerant 38 passes through the core portion of the evaporator, for example, a two-pass system, a three-pass system, and so forth.
FIG. 2 shows a perspective view of an exemplary configuration of evaporator 32. Evaporator 32 includes a refrigerant inlet 42, a refrigerant outlet 44, and a plurality of tube sheet assemblies 46 arranged in a stacked or back-to-back manner and brazed together to form the central portion, or core, of evaporator 32. Tube sheet assemblies 46 are operatively connected at their upper ends by an inlet conduit 48 and an outlet conduit 50 (each of which are shown in ghost form). Tube sheet assemblies 46 are further operatively connected at their lower ends by a first intermediate conduit 52 and a second intermediate conduit 54 (each of which are shown in ghost form). Conduits 48, 50, 52, and 54 will be discussed in greater detail below. Tube sheet assemblies 46 are arranged to define spaces 56 therebetween to accommodate fins 58. Fins 58 operate to increase the heat transfer performance of evaporator 32, as known to those skilled in the art.
Each of tube sheet assemblies 46 includes a pair of tube plates arranged in a face-to-face manner and brazed together about their periphery. A cavity (not shown) is formed between the brazed tube plates through which refrigerant 38 flows. Evaporator 32 includes two types of tube sheet assemblies 46, straight tube sheet assemblies 60 and U-turn tube sheet assemblies 62.
Referring to FIGS. 3-4 in connection with FIG. 2,
FIG. 3 shows a planar view of a first tube plate 64 of one of straight tube sheet assemblies 60. FIG. 4 shows a planar view of a second tube plate 66 of one of U-turn tube sheet assemblies 62. It should be noted that one of straight tube sheet assemblies 60 is formed by a pair of first tube plates 64. Similarly, one of U-turn tube sheet assemblies 62 is formed by a pair of second tube plates 66. First and second tube plates 64 and 66, respectively, are provided to illustrate the intended flow of refrigerant 38 through their corresponding straight and U-turn tube sheet assemblies 60 and 62, respectively.
Referring particularly to FIG. 3, first tube plate 64 includes a first fluid flow section 68 in fluid communication with each of inlet conduit 48 and first intermediate conduit 52. First tube plate 64 further includes a second fluid flow section 70 in fluid communication with each of outlet conduit 50 and second intermediate conduit 54. A first dividing wall 72 separates first and second fluid flow sections 68 and 70, respectively. As such, when a pair of first tube plates 64 are brazed together, refrigerant 38 flowing in first fluid flow section 68 cannot mix with refrigerant 38 flowing in second fluid flow section 70.
Refrigerant 38 flows into first fluid flow section 68 of straight tube sheet assembly 60 from inlet conduit 48 and exits first fluid flow section 68 via first intermediate conduit 52. In contrast, refrigerant 38 flows into second fluid flow section 70 of straight tube sheet assembly 60 from second intermediate conduit 54 and exits via outlet conduit 50.
Referring now to FIG. 4, second tube plate 66 includes a third fluid flow section 74 in fluid communication with first intermediate conduit 52. Second tube plate 66 further includes a fourth fluid flow section 76 in fluid communication with second intermediate conduit 54. A second dividing wall 78 partially separates third and fourth fluid flow sections 74 and 76, respectively. In addition, a third dividing wall 80 separates third and fourth fluid flow sections 74 and 76 from inlet and outlet conduits 48 and 50, respectively. As such, when a pair of second tube plates 66 are brazed together, refrigerant 38 flows from third fluid flow section 74 into fourth fluid flow section 76. However, this refrigerant 38 cannot mix with refrigerant 38 flowing in inlet and outlet conduits 48 and 50.
Refrigerant 38 flows into third fluid flow section 74 of U-turn tube sheet assembly 62 from first intermediate conduit 52. Refrigerant 38 subsequently flows from third fluid flow section 74 into fourth fluid flow section 76, and exits fourth fluid flow section 76 via second intermediate conduit 54.
FIG. 5 shows a phantom schematic representation of evaporator 32 illustrating a pre-determined flow path 79 of refrigerant 38 from refrigerant inlet 42, through evaporator 32, to refrigerant outlet 44 of evaporator 32. As shown, refrigerant 38 enters evaporator 32 via refrigerant inlet 42, and flows in inlet conduit 48 to straight tube sheet assemblies 60. Refrigerant 38 flows through first fluid flow section 68 (FIG. 3) of each of straight tube sheet assemblies 60, where refrigerant 38 enters first intermediate conduit 52 (FIG. 2). Refrigerant 38 then flows through first intermediate conduit 52 to U-turn tube sheet assemblies 62, enters third fluid flow section 74 (FIG. 4), and flows into fourth fluid flow section 76 (FIG. 4). Refrigerant 38 subsequently flows out of U-turn tube sheet assemblies 62 into second intermediate conduit 54 (FIG. 2), and enters second fluid flow section 70 (FIG. 3) of straight tube sheet assemblies 60. Refrigerent 38 flows from second fluid flow section 70 into outlet conduit 50 (FIG. 2), where it exits evaporator 32 via refrigerant outlet 44.
Evaporator 32 represents a multiple pass flow through a central core 82 of straight and U-turn tube sheet assemblies 60 and 62, respectively. This multiple pass flow technique facilitates optimal cooling performance of evaporator 32. Unfortunately, the cooling performance of evaporator 32 may be compromised when a bypass situation occurs in core 82. The bypass situation occurs when the flow of refrigerant 38 deviates from its pre-determined flow path 79. That is, refrigerant 38 is able to bypass into another section of core 82, instead of being directed through evaporator 32 in the pre-determined, designed, or expected manner.
Internal leakage, or bypass, in a heat exchanger can be caused by a faulty manufacturing technique. For example, the incomplete brazing of first tube plates 64 (FIG. 3) that form straight tube sheet assemblies 60 and/or second tube plates 66 (FIG. 4) that form U-turn tube sheet assemblies 62, can result in defects in the internal bracing of core 82 that lead to the bypass situation. Some examples of bypass include, but are not limited to, leakage between first and second fluid flow sections 68 and 70 (FIG. 3), respectively, leakage between inlet and outlet conduits 48 and 50 (FIG. 2), respectively, leakage between first and second intermediate conduits 52 and 54 (FIG. 2), respectively, and so forth.
Some manufacturing facilities perform validation testing, or audit checks, to identify defective heat exchanger cores prior to their entry into the market. One such test attempts to identify internal leakage, or bypass, by measuring a quantity of heat rejection for a heat exchanger core and comparing that measured quantity of heat rejection with a desired heat rejection threshold. Unfortunately, such testing is complex, time consuming, and prone to error.
Obviously, it is highly desirable to prevent defective heat exchangers from entering the market. Moreover, as the complexity of heat exchanger increases, through more complex fluid flow paths such as heat exchanger 32, so too does the probability of internal leakage. Accordingly, what is needed is an efficient method and a cost effective system for inspecting heat exchangers for internal leakage.